Thousands
of years ago, an ancient Mesoamerican society carved sculptures from the
green-blue rock jade. Although recent expeditions to the region turned
up the jades geologic source  deep inside a fault  only
now have scientists begun to unlock the clues inside the stone that provide
a rare look at conditions deep inside Earth.

To learn about the history of fluid inside a subduction zone, most researchers
previously relied on indirect evidence such as the end-product rocks at
Earths surface, which have been physically altered by volcanic activity.
But Sorena Sorensen, a geochemist at the National Museum of Natural History
in Washington, D.C., and colleagues say that jade, which forms from fluid
tens of kilometers below Earth, provides a direct look at the fluids
source and chemical evolution as it moved through rock.

When struck with a beam of electrons,
jade luminesces in colors that represent its composition. Researchers
can use that information to learn about the fluid deep inside subduction
zones. Image is by Sorena Sorensen.

Until now, researchers were speculating what happens at depth,
says Virginia Sisson, a jade expert associated with the American Museum
of Natural History (AMNH) in New York City. Here, we actually have
an example of whats happening, she says. Because fluids at
depth directly affect the shaping of features at Earths surface,
this is a big deal for people interested in the cycling of elements,
as well as what goes on in the mantle and making volcanoes, she
says.

The heart of Sorensens teams investigation goes back 3,000
years to Guatemala, where people of the Olmec culture sculpted the regions
rare blue-green rock. In 2001, expeditions, of which Sisson was part,
to the region culminated in the discovery of jade-bearing rocks the size
of houses (see Geotimes, August 2002).

Following the discovery of the Guatemalan jade source, researchers started
looking into how it formed. Most scientists agreed that it formed within
the regions Motagua Fault, but details about how it formed remained
a mystery.

Sorensen and colleagues new research, published in the July American
Mineralogist, provides evidence that jade forms when hydrothermal
fluids stream through open cracks and deposit the minerals requisite
elements within massive veins. Sorensen looked at the microscopic structure
of hundreds of samples from the Guatemalan region and five other jade-producing
regions around the world. Bombarding the samples with a stream of electrons
causes jade to luminesce spectacularly, Sorensen says, in
colors ranging from blue and red to yellow and green, which reflect various
stages of jades chemical formation.

At the earliest stage in subduction zones, elements combine and crystallize
into blue and red luminescing jade. As new elements enter the mix, the
jade becomes yellow-green. The final crystallizing stage (not all make
it that far) forms the rare, yet most familiar, green jade. By combining
observations of these colors, which indicate the presence of specific
elements, with oxygen isotope analysis, Sorensen was able to hone in on
possible sources of jade-creating fluid.

The analysis revealed different fluids that correspond to three different
sources: one apparently altered by seawater, another that stems from rocks
altered by a high-temperature hydrothermal system, and another that comes
from a mantle source. Sorensens research shows that a lot
of elements that people think dont really move, are moving,
Sisson says.

Still unknown, however, are details about the precise depth of jade formation,
Sisson says. Another question remains about how the jade, once formed,
actually makes it from the depths of subduction zones to Earths
surface.

The original team that discovered the jade would like to return to Guatemala
to investigate reports of jade beyond the currently known extent of about
150 kilometers along the Motagua Fault. Its just really widespread,
Sisson says. Every time we go down there, we find new [deposit]
localities farther and farther away from the main source.